In wireless communication networks, a way to increase both coverage and capacity is to use coordination for signal transmission and reception between the nodes in the network and wireless devices located in the coordinating coverage areas of the network nodes. This is generally referred to as Coordinated Multi-Point, CoMP, transmission and reception. This coordination may be used in downlink communication for scheduling and data transmission using, for example, beam forming or joint coherent processing and transmission, or in uplink communication, where a multitude of antennas are used to suppress and cancel interference and increase the signal-to-noise ratio, SNR.
In downlink communication, a CoMP system may also be seen as geographically distributed multiple transmission points over the system's coverage area which perform cooperative transmission. By allowing this coordination, coordinated transmission and reception strategies among network nodes, such as, e.g. adjacent Evolved NodeBs, eNodeBs, may be applied in order to coordinate the resources usage and manage interference.
One example of a CoMP scenario is shown in FIG. 1-2.
FIG. 1 depicts a communications network 100 comprising a number of network nodes 101, 102, 103. The network nodes 101, 102, 103 are configured to provide access to the communications network 100 over a radio link to wireless devices in their corresponding cells. The network nodes 101, 102, 103 may be connected and configured to communicate with each other over, e.g. an X2 connection 104.
FIG. 2 depicts a part of the wireless communications network 100 comprising the cells 201, 202, 203, 204, 205, 206, 207, 208, 209 of the network nodes 101, 102, 103. In FIG. 2, the cells 203, 206, 209 are comprised in a CoMP cell of cooperating cells of the access network nodes 101, 102, 103. The CoMP cell is shown as marked with vertical lines in FIG. 2. A CoMP cell may be described as a collection of cooperating cells of one or more network nodes in which signals of multiple antennas may be combined to form a joint coordinated transmission or reception of data to or from wireless devices served in the cooperating cells. In FIG. 2, this means that, within the cells 203, 206, 209, i.e. the CoMP cell, the network nodes 101, 102, 103 may perform coordinated data transmission or reception to or from wireless devices served by the cells 203, 206, 209.
In a CoMP cell, Channel State Information, CSI, is estimated by the wireless devices and reported to the network node via feedback channels. The network node may then use the CSI and the distributed antenna array of the CoMP system for applying different Radio Resource Allocation, RRA, strategies. A RRA strategy may comprise spatial precoding and multi-user multi-cell scheduling in the joint data transmissions to the wireless devices. This may be performed in order to mitigate intra-cell, as well as, inter-cell interference and efficiently separate streams intended to different wireless devices.
While the spatial multiplexing of signals intended to different wireless devices is done using spatial precoding, spectral efficiency gains are often obtained by transmitting to spatially compatible wireless devices, i.e. a given group of wireless devices whose channels are favourable for spatial separation, using multi-user multi-cell scheduling.
It may be shown that there are significant joint transmission gains that may be obtained with using multi-user multi-cell scheduling within a CoMP scenario, such as, e.g. an adaptive Space Division Multiple Access, SDMA, scheduling, as compared to single-user single-cell scheduling.
However, it may be difficult to select which wireless devices the multi-user multi-cell scheduling algorithm should be applied to, i.e. the wireless devices that may efficiently share the same resources in space; or, in other words, selecting groups of wireless devices with orthogonal channels that may be co-scheduled for the CoMP transmission.
For example, in some CoMP scenarios, the scheduling algorithm may be applied to all wireless devices in a system's coverage area which perform cooperative transmissions. By performing this type of wide search, the scheduling algorithm may find the best wireless devices to co-schedule on the same non-orthogonal spatial resource. However, this approach results in a high degree of scheduling complexity, since it requires very extensive pre-coder calculations, e.g. the pre-coder vectors and/or effective channel gains for each candidate wireless device have to be repeatedly calculated. Thus, performing a wide search in a large CoMP scenario within the required time frame may even be impossible if there are many wireless devices to be scheduled or several candidate wireless devices in the CoMP scenario. The required time frame may here be for every Transmission Time Interval (TTI). One TTI may here correspond to 1 ms as defined in the LTE standard.
In a large CoMP scenario, instead of having joint data transmissions being performed within the entire large CoMP scenario, the joint data transmissions may be performed in different clusters of cells within the large CoMP scenario. This may be performed in order to e.g. reduce the scheduling complexity in the large CoMP scenario. The clusters of cells may be described as subsets of cells that are mutually exclusive, i.e. each subset of cells comprises cells that do not occur in any other subset of cells.
These approaches may make use of Reference Signal Received Power (RSRP) measurements of all wireless devices for all the cells belonging to its large CoMP scenario to dynamically create the clusters of cells within the entire large CoMP scenario, which may be adapted to current channel and load conditions. Thus, the wireless devices in each cluster of cells may be jointly scheduled by a scheduling algorithm, e.g. an SDMA algorithm. This may be useful when the backhaul or other hardware limitations makes joint coordinated transmission or reception with all antennas in the entire large CoMP scenario unfeasible.
This means that coherent CoMP transmissions will be performed in smaller areas with fewer involved cells and antennas than in the large CoMP scenario. Also, the scheduling algorithms applied to these smaller areas will have a lower scheduling complexity than a scheduling algorithm applied in the large CoMP scenario since these areas comprises fewer wireless devices.
However, there is a significant loss of the system spectral efficiency as the number of clusters of cells is increased when comparing to the no-clustering scenario, i.e. the large CoMP scenario. This degradation on the system performance is due to the generation of more interference for each additional cluster of cells following the subsequently increased cluster-edge length where coherent CoMP may not be utilized; this, besides also limiting the coordination degree.